Code-division multiple access (CDMA) communication systems are well known. One exemplary CDMA communication system is the so-called IS-95 which is defined for use in North America by the Telecommunications Industry Association (TIA). For more information on IS-95, see TIA/EIA/IS-95, Mobile Station-Base-station Compatibility Standard for Dual Mode Wideband Spread Spectrum Cellular System, January 1997, published by the Electronic Industries Association (EIA), 2001 Eye Street, N.W., Washington, D.C. 20006. A variable rate speech codec, and specifically Code Excited Linear Prediction (CELP) codec, for use in communication systems compatible with IS-95 is defined in the document known as IS-127 and titled Enhanced Variable Rate Codec, Speech Service Option 3 for Wideband Spread Spectrum Digital Systems, September 1996. IS-127 is also published by the Electronic Industries Association (EIA), 2001 Eye Street, N.W., Washington, D.C. 20006.
In modern CELP decoders, there is a problem with maintaining high quality speech reproduction at low bit rates. The problem originates since there are too few bits available to appropriately model the "excitation" sequence or "codevector" which is used as the stimulus to the CELP synthesizer. Thus, a need exists for an improved method and apparatus which overcomes the deficiencies of the prior art.